[FR] Public API to Detect FieldValue.increment()

The Firebase Admin Node SDK currently does not provide a public API to detect if a value is a FieldValue.increment() object. This forces developers to rely on internal implementation details like checking for constructor.name === 'NumericIncrementTransform', which is fragile and may break with SDK updates.

Current Workaround

// Current approach relies on internal implementation
function hasFieldValueIncrement(value: unknown): boolean {
  return (
    typeof value === 'object' &&
    value.constructor &&
    value.constructor.name === 'NumericIncrementTransform'
  );
}

Problem

1. Fragility: The NumericIncrementTransform class name is an internal implementation detail
2. No Stability Guarantee: This approach may break in future SDK versions
3. Lack of Documentation: No official way to detect increment operations

Requested Enhancement

Please provide a public API method to detect FieldValue.increment() objects, similar to how other Firebase features provide type guards.

Suggested API

import { FieldValue } from 'firebase-admin/firestore';

// Option 1: Static method on FieldValue
FieldValue.isIncrement(value: unknown): value is FieldValue

// Option 2: Dedicated type guard function
import { isIncrement } from 'firebase-admin/firestore';
isIncrement(value: unknown): value is FieldValue

Use Case

This is needed for:

• Data validation pipelines that need to identify increment operations
• Custom diffing systems that handle FieldValue operations differently
• Development tools that analyze Firestore update patterns
• Testing utilities that need to verify increment operations